1. Field of the Invention
The present invention is directed to an improved apparatus and method for firing green ceramic articles, such as chip capacitors, which include a ceramic dielectric such as barium titanate.
2. The Prior Art
As conducive to an understanding of the present invention, it may be stated that the current methods for manufacturing monolithic ceramic capacitors typically involve forming ceramic sub-strates from fine particles of the desired dielectric composition in a binder of organic polymeric material. The sub-strates may be imprinted, as by silk screening with an ink incorporating a binder and finely divided particles of metal resistant to oxidation at sintering temperatures to form an electrode area. A plurality of electrode bearing sub-strates of green ceramic are superimposed in a carefully oriented manner, such that the electrodes in successive layers are arrayed in partial registering position and are then compressed.
Typically, the main body portion of electrodes in adjacent layers will overlap throughout their major extent, with the margins of electrodes in alternate layers being offset.
Thereafter, the superimposed sub-strates are subdivided into small increments along cutting lines such as to expose edge portions of the electrodes of alternate layers at opposite sides of the increments.
The increments are heated to firing temperature to drive off the binder and sinter the dielectric to thus convert the green ceramic to a dense monolithic material having a high dielectric constant.
Finally, the fired capacitors thus formed are terminated, i.e. fitted with leads, and, typically, thereafter encapsulated in an insulating jacket.
The operation of prefiring the green ceramic articles to burn off organic materials and subsequent firing, has heretofore been protracted, energy intensive, costly, and has involved a great deal of handling of the fragile and often miniscule prefired articles. More particularly, in accordance with a conventional manufacturing procedure, the articles are carefully loaded onto individual sleds in mutually spaced relation. The sleds, which are mounted on conveyors, are thereafter led through horizontal kilns at a controlled rate.
Often the firing and cooling period will take approximately 17 hours. The kilns typically are expensive and extremely inefficient in the use of energy.
More recently an improved procedure has been developed whereby the green ceramic preforms are loaded on an aluminum tray and heated to drive off the organics, the articles, after burnout, being loaded into boats in bulk, covered with zirconium oxide sand, and then fired. This improvement has greatly increased the firing rate but has, nonetheless, required the use of inefficient kiln furnaces and the like.